The present invention relates to a film formation method and, more particularly, to a method of forming a metal film used in wirings or the like of a semiconductor device.
In the manufacture of a semiconductor integrated circuit, a method of forming a metal wiring by plating has been studied.
When forming a metal film by plating, the metal film is strongly influenced by a liner in the initial stage of film formation. A case will be described wherein a copper film is to be formed in accordance with electroplating by using a titanium nitride film as a liner film. When a 500 nm thick copper film is formed on a 200 nm thick titanium nitride film, the crystal grain size of copper is about 100 nm. The reason why the plating film has a small crystal grain size is supposed to be as follows.
In the initial stage of film formation, a large number of nuclei are formed on the surface of the liner film. Crystal growth of the plating metal is started from these nuclei. Accordingly, mutual crystal growth interferes with each other to reduce the crystal grain size of the metal plating film.
When a metal plating film having a small crystal grain size is used for a wiring or the like, the electromigration resistance degrades since electromigration tends to progress along the grain boundary. Therefore, with the conventional method, it is difficult to form a metal plating film having an excellent electromigration resistance and large crystal grain size.
As a wiring formation process for a semiconductor integrated circuit, a wiring metal filling process (the so-called damascene method) has been used widely. According to this technique, a metal film is filled in a wiring groove (or a wiring groove and a connection hole) formed in an interlayer insulating film, and then an excessive metal film is removed by polishing. Plating is studied as a promising method in the damascene method as well.
A process of forming a copper plating film in the damascene method is as follows. First, a barrier metal film and a copper film are formed in a wiring groove and a connection hole formed in an interlayer insulating film. These films serve as an electric current supplying layer. The electric current supplying layer forms a conductive film for introducing a current during electroplating. After that, a copper film is formed on the electric current supplying layer by electroplating. When forming the copper film on the electric current supplying layer by electroplating, a sufficiently large amount of electrons must be supplied to the plating solution (copper ions in the plating solution) even during the initial stage of electroplating. Hence, the electric current supplying layer has a conductivity to a certain degree or more, in other words, a minimum film thickness.
When the wiring groove or connection hole is micropatterned, the method described above suffers the following problems. Sputtering is generally used to form the electric current supplying layer. Sputtering has poor step coverage. Accordingly, the film thickness decreases near the bottoms of the groove and hole, and overhangs are formed near the upper portions of the groove and hole. When a sufficiently thick electric current supplying layer is to be assured near the bottoms of the groove and hole, the overhangs increase near the upper portions of the groove and hole. When the overhang increases, penetration of the plating solution is interfered with. As a result, a void or seam is formed in the plating film, making it difficult to form a metal plating film having high quality.
When a metal plating film having a void or seam is used for a wiring or the like, the electromigration resistance degrades because electromigration is promoted by the void or seam. Therefore, with the conventional method, it is difficult to form a high-quality metal plating film having good electromigration resistance in a groove or hole.
It is the first object of the present invention to provide a film formation method for the manufacture of a semiconductor device, which can form a metal film having a large crystal grain size to provide good electromigration resistance.
It is the second object of the present invention to provide a film formation method for the manufacture of a semiconductor device, which can form a high-quality metal film in a groove or hole without defects such as void or seam to provide good electromigration resistance.
According to the first aspect of the present invention, there is provided a film formation method for manufacture of a semiconductor device, comprising the steps of: forming a first metal film as a continuous film on a substrate; forming a second metal film as discontinuous films on the substrate formed with the first metal film; and forming a third metal film by plating on the substrate formed with the first and second metal films.
According to the present invention, the second metal film as the discontinuous metal films (from another viewpoint, the second metal film formed as discrete films) serves as nuclei when forming the third metal film. The third metal film preferentially grows from the nuclei. Accordingly, a metal plating film (third metal film) having a large crystal grain size can be formed by controlling the nucleus density. For example, when a metal plating film having a thickness of about 0.1 xcexcm to 1 xcexcm is formed, a metal plating film having high quality can be obtained. As a result, the electromigration resistance of the metal film can be improved.
In order to allow the second metal film formed as discrete films to serve as the nuclei in formation of the third metal film, the first, second, and third metal films are preferably made of metal materials selected such that the third metal film is more preferentially formed on the second metal film than on the first metal film.
In particular, the second and third metal films are preferably made of the same metal material. This is because, when the crystal state (the lattice constant and the like) of the metal that forms the third metal film is identical or similar to that of the second metal film, the crystallinity or degree of purity of the third metal film can be improved. When the lattice constants of the metals constituting the two metal films largely differ from each other, the resistivity of the third metal film becomes high, and the resistivity within the substrate surface varies largely.
Discontinuity of the second metal film can be obtained by depositing a metal material on a step portion of the substrate formed with the first metal film. In particular, the metal material is preferably deposited by sputtering.
The method may further comprise a step of causing an alloy reaction between a metal constituting the first metal film and a metal constituting the second metal film prior to formation of the third metal film. This makes it possible to obtain high adhesion strength between the first and second metal films and achieve highly reliable film formation.
As plating for forming the third metal film, electroplating, electrolessplating, substitution plating, and the like can be used. In particular, electroplating or electrolessplating is preferably used. When forming the third metal film by electroplating, the first metal film may be used as an electrode.
The first metal film is preferably formed on the substrate formed with at least one of a groove and a hole.
According to the second aspect of the present invention, there is provided a film formation method for manufacture of a semiconductor device, comprising the steps of: forming a first metal film that serves as a seed in electrolessplating on a substrate formed with at least one of a groove and a hole; forming a second metal film by electrolessplating on the substrate formed with the first metal film; and forming a third metal film by electroplating on the substrate formed with the first and second metal films.
The first, second, and third metal films may be appropriately selected. The first and second metal films are preferably made of the same metal material. Alternatively, the second and third metal films are preferably made of the same metal material. In particular, the first, second, and third metal films are preferably made of the same metal material.
FIG. 9 shows the measurement result indicating the thickness of the second metal film (in the case of FIG. 9, a copper film formed by electrolessplating for 10 minutes) with respect to the thickness of the first metal film (in the case of FIG. 9, a copper film formed by sputtering). In the case of FIG. 9, under the first metal film, a barrier metal film is formed on a silicon oxide film.
As is apparent from FIG. 9, when the first metal film does not exist or is very thin, the second metal film is not substantially formed. When the thickness of the first metal film becomes almost 3 nm or more, film formation of the second metal film progresses drastically. This may be because the first metal film serves as the seed that promotes growth of the second metal film formed by electrolessplating. Note that the seed means that the first metal film serves as the source that promotes growth of the second metal film.
Therefore, if the first metal film is formed in advance, the second metal film can be effectively formed by electrolessplating. Since the second metal film is formed by electrolessplating, it has excellent step coverage, and any overhang is rarely formed in a groove or hole. Therefore, when the third metal film is formed on the second metal film by electroplating, the third metal film can be uniformly filled in the groove or hole.
In this manner, according to the present invention, a metal film having high quality can be obtained, and the electromigration resistance of the metal film can be improved.
The first metal film preferably has a thickness of 3 nm or more. Normally, the thickness of the first metal film is preferably set to 3 nm or more throughout the entire film formation region on the substrate. It is confirmed that, if the thickness of the first metal film is about 5 nm or more, the adhesion properties with the barrier metal are improved. From this viewpoint, the thickness of the first metal film is preferably about 5 nm or more.
FIG. 10 shows the measurement result indicating the surface roughness of the third metal film with respect to the thickness of the first metal film. FIG. 10 shows a case wherein copper is used to form the first, second, and third metal films, and a case wherein silver is used to form the first, second, and third metal films. As is apparent from FIG. 10, when the thickness of the first metal film becomes almost 70 nm or more, the surface roughness of the third metal film degrades quickly. Therefore, this value can be regarded as the upper limit of the thickness of the first metal film.
From the foregoing, the first metal film preferably has a thickness d that falls within a range of 3 (nm)xe2x89xa6d (nm)xe2x89xa670 (nm). Normally, the thickness d is preferably set to fall within this range throughout the entire film formation region on the substrate.
The surface resistance obtained after formation of the second metal film will be described. In electroplating, a relatively negative potential is applied to a substrate in an electrolyte to have positive metal ions attached to the surface of the substrate. For this purpose, a conductive layer that can supply a sufficiently large current is necessary. FIG. 11 shows the current density of electroplating with respect to the surface resistance (sheet resistance) of an electric current supplying layer (in the case of FIG. 11, barrier metal film+first metal film (copper film formed by sputtering)+second metal film (copper film formed by electrolessplating)). As is apparent from FIG. 11, to grow a metal film by electroplating, the substrate formed with the first and second metal films preferably has a surface with a sheet resistance of about 0.4xcexa9 or less before formation of the third metal film. Normally, the sheet resistance is preferably set to 0.4xcexa9 or less throughout the entire film formation region on the substrate. If the sheet resistance exceeds this value, initial electroplating does not occur (or is very difficult to occur), making it very difficult to fill the third metal film in the groove or hole.
After formation of the first metal film before formation of the second metal film, or after formation of the second metal film before formation of the third metal film, denatured layers composed of an oxide or the like may be undesirably formed at the interfaces among these metal films. To prevent this, the surface of the first metal film may be etched after formation of the first metal film before formation of the second metal film. Alternatively, the surface of the second metal film may be etched after formation of the second metal film before formation of the third metal film. From the viewpoint of suppressing formation of the denatured layer, the substrate is not preferably exposed to the atmosphere after formation of the second metal film is started before formation of the third metal film is ended.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.